External Transport Barrier Research Articles

Statistical analysis of magnetic divertor configuration influence on H-mode transitions

DIII-D plasmas are compared for two upper divertor configurations: with the outer strike point on the small angle slot (SAS) divertor target and with the outer strike point on the horizontal divertor target (HT). Scanning the vertical distance between the magnetic null point and the divertor target over a range 0.10–0.16 m is shown to increase the threshold power, Pth , and edge plasma power, PLoss , for the low-to-high confinement (L–H) and H–L transitions respectively, by up to a factor of 1.4. The X-point height scans were performed at three L-mode core plasma line average electron densities, n¯e= 1.2, 2.2 and 3.6 ×1019m−3 , to investigate the density dependence of divertor magnetic configuration influence on Pth . The X-point height, Zx-pt , was further extended across the range 0.16–0.22 m with the more open HT divertor configuration, for which a clear decrease in Pth with increasing Zx-pt is observed. The dependence of Pth on divertor magnetic geometry is further investigated using a time-dependent probability density function (PDF) model and information geometry to elucidate the roles played by pedestal plasma turbulence and perpendicular velocity flows. The degree of stochasticity of the plasma turbulence is observed to be sensitive to the plasma heating rate. The calculated square of the information rate shows changes in the relative density fluctuations and perpendicular velocity PDFs begin 2–5 ms prior to the L–H transition for three plasmas; providing a crucial measurement of the dynamic timescale of external transport barrier formation. Additionally, both information length and rate provide potential predictors of the L–H transition for these plasmas.

High-βp scenario realized by the integration of internal and external transport barriers in the HL-2A tokamak

High-βp scenario addresses needs for an attractive tokamak fusion reactor design, where βp is the normalized plasma poloidal beta. High-βp experiments have been performed during recent years in the HL-2A tokamak. The high-performance region is realized by the integration of internal and external transport barriers which are dubbed as ITBs and ETBs, namely, double transport barriers (DTBs), with the high-power NBI heating. Generally, the ITB forms and becomes strong after the NBI injection on HL-2A. Subsequently, the edge ion temperature and toroidal rotation increase, as a result that the ETB or pedestal creates and L-H transition occurs. A high-βp (βp∼3) scenario is obtained with Ip∼110 kA and q95∼5, and another high-βp (βp∼2.7) scenario is also realized with Ip∼160 kA and q95∼4.2. The two high-βp scenarios are both characterized by a large-radius ITB and an ETB. However, the two scenarios are different. For the former there are strong large-radius ITB and weak ETB on radial ion temperature profiles, but for the later there are weak large-radius ITB and strong ETB. Although two high-βp scenarios on HL-2A have been accessed, however they are both transient. In addition, MHD instabilities in high-βp scenarios are also present for a high fusion gain and reactor relevant high fusion-density plasma.

Modeling of the transition mechanism from electrostatic drift-type modes to electromagnetic kinetic ballooning mode-dominant regime in high poloidal-beta discharges

Recent analyses (for example, see the paper by Staebler et al (2018 Phys. Plasmas 056113)) indicate that in DIII-D high poloidal-beta discharges turbulent transport, particularly for ions, is governed by the electromagnetic kinetic ballooning mode (KBM) in most radial regions, including the outer core between the internal and external transport barriers (ITBs and ETBs). Considering that in the usual L- or H-mode plasmas turbulent transport is widely believed to be dominated by the electrostatic drift-type modes, such as the ion temperature gradient (ITG) or trapped electron mode (TEM), a modeling study is presented to show how this transition of dominant modes can occur. In the ITB region, where there already exist several theoretical models, the focus is put on clarifying the relative role between the Shafranov shift and linear electromagnetic effects. While these two can play an important role in the transition or ITB formation by stabilizing the electrostatic drift-type modes, a significant difference is found in that the former mainly works on the TEM, while the latter works on the ITG. In contrast to the ITB region, in the outer core between the ITB and ETB the transition appears to be more relevant to the destabilization of the KBM itself, rather than the stabilization of electrostatic modes. A jump of edge plasma beta through the pedestal formation is shown to play the critical role in this destabilization by making the KBM threshold temperature gradient smaller than that of the ITG/TEM, thus allowing an earlier excitation of KBM when background temperature gradients increase by external heating.

Mechanisms and dynamics of the external transport barrier formation in non-linear plasma edge simulations

L-H transition features are reproduced using three-dimensional first-principles plasma edge turbulence simulations. A transport barrier is observed to form spontaneously above a threshold of the input power. The physical mechanism relies on the coupling between the equilibrium pressure gradient and the poloidal flow, through both the radial force balance and the neoclassical friction. Accounting for the actual radial profile and time evolution of the latter is key to the barrier formation. It is found that neoclassical friction acts as an energy source for the flow, which largely overcomes the sink due to the turbulent Reynolds stress during the whole barrier lifetime. Importantly, experimentally reported dynamical features are recovered during the formation and lifetime of the barrier. This includes dithering of the radial electric field, which is reminiscent of experimentally observed limit-cycle oscillations and quasi-periodic relaxation oscillations showing similarities with type-III ELMs. These rich dynamics emerge from interplay between turbulence, turbulence-driven flows and the equilibrium flow governed by force balance.

Modeling of the edge plasma of MAST Upgrade with a Super-X divertor including drifts and an edge transport barrier

The Super-X divertor edge plasma of the future MAST Upgrade tokamak (Lisgo et al 2009 36th EPS Plasma Physics Conf. 33E O4-046, Katramados et al 2011 Fusion Eng. Des. 86 1595, Morris 2012 IEEE Trans. Plasma Sci. 40 682) was simulated with the B2SOLPS5.2 transport code including, for the first time, the effects of drifts due to electric field and magnetic field gradients. The previous simulations did not contain drift effects as well as an account of the external transport barrier. The expected reduction in temperatures and heat flux densities at the low-field side divertors was obtained in the simulations. However, an account of drifts and parallel currents led to an up–down asymmetry of the power to the plates in the connected double null configuration, which was not observed in the simulations without drifts.

Characteristic properties of fluctuating particle fluxes near the last closed flux surface in the course of lower hybrid heating and during a transition into the improved confinement mode in the FT-2 tokamak

Results are presented from experimental studies of the time behavior of the transport processes in the edge plasma of the FT-2 tokamak during auxiliary lower hybrid heating when an internal transport barrier and then an external transport barrier form in the plasma. An analysis of the data on turbulent particle transport in the edge plasma shows that the radial electric field generated inside the plasma column during auxiliary heating plays an important role in both the formation of a transport barrier and the suppression of anomalous transport at the plasma periphery in the postheating phase of the discharge. The mechanism for the formation of a negative radial electric field E r near the last closed flux surface after the end of the lower hybrid heating pulse is considered. Fluctuation spectra of the particle density and poloidal electric field are presented that characterize the process of suppression of microturbulence at the plasma periphery. The experimental data were obtained with the use of movable multielectrode Langmuir probes.

H-mode and transport barriers in helical systems**Invited paper.

This paper presents the physics of two bifurcations in confinement of helical devices—(1) to the H-mode and (2) to internal transport barrier (ITB)-like electron temperature profiles as they develop under neoclassical electron root conditions in 3-dimensional systems. With their characteristics—low or negative magnetic shear, strong toroidal flow damping, experimental variability of poloidal flow damping, radial electric field enforced by ambipolarity, diagnostic access to sophisticated spatial and temporal structures of turbulence thanks to low-power operation with external confinement—helical devices provide unique contributions to the physics of transport barriers. The bifurcation to confinement with external transport barrier seems to be soft and the leading role of the electric field gradient is confirmed; the one to ITB-like core profiles is a hard transition and it is the electric field which governs it. The paper summarizes the status of H-mode research in helical systems and discusses the impact of the electron root on core confinement.

Edge localized modes control by stochastic magnetic fields

This paper describes a new approach for modelling the pedestal energy transport in the presence of a small radial magnetic perturbation. The cases of a ballooning instability leading to Type I edge localized modes (ELMs) and a magnetic perturbation generated by external coils are treated. The model for Type I ELMs is based on the linear ideal MHD code MISHKA coupled with the non-linear energy transport code TELM in a realistic tokamak geometry. The main mechanism of the increased transport through the external transport barrier in this model of ELMs is due to the appearance of a radial velocity and a radial magnetic field perturbation due to the MHD mode. Both lead to additional transport perpendicular to the magnetic surface and hence to a relaxation of the pressure profile in the unstable zone. The typical Type I ELM time-cycle was reproduced numerically including the destabilization of the ballooning modes leading to the fast (250 μ s) collapse of the pedestal pressure followed by the edge pressure profile re-building on a diffusive time scale. A possible mechanism for the control of Type I ELMs using a stochastic plasma boundary created by external coils is modelled in this paper using data on ELM suppression by I-coils from the DIII-D experiment. In the stochastic layer the transverse transport is effectively increased by diffusion of the magnetic field lines. The modelling results demonstrate the possibility of decreasing the edge pressure gradient to a value that is just below the ideal ballooning limit, leading to a high confinement regime without Type I ELMs.

Transport and turbulence studies in the T-10 tokamak

Transitions to the improved confinement regimes were studied in the T-10 tokamak with electron cyclotron resonance heating (ECRH). The H-mode regime with and without internal transport barrier (ITB) was obtained and studied. The H-mode was obtained by power increase, pellet injection and biasing. The confinement time in the pellet enhanced confinement regime is 30% higher than that in the spontaneous H-mode. In biasing experiments an enhancement factor of up to 1.55 was obtained for the energy confinement time. It was shown that subtle changes in the safety factor q profile allow us to obtain either ITB or external transport barrier only, or both of them, other conditions being approximately similar. The global confinement in the regime with the density above the Greenwald limit under strong gas-puffing and ECRH was investigated. To identify the type of turbulence responsible for transport, the Ohmic and ECRH regimes with variation of safety factor at the limiter qL and gas puffing were considered. Two regions were observed along the minor radius with different turbulence properties.

Dynamics of the transport barrier formation on the FT-2 tokamak caused by lower hybrid heating

Experiments at the FT-2 tokamak had demonstrated effective plasma LH heating, which was accounted for by both direct absorption of RF power and plasma transport suppression. The improved core confinement accompanied by Internal Transport Barrier (ITB) formation was observed. The RF pulse switch off is followed by triggering of LH transition and the External Transport Barrier (ETB) formation near the last closed flux surface. The present paper is devoted to a much more detailed study of the radial electric fieldEr behaviour in the region of ITB and ETB and its influence on the tokamak microturbulence in these regions. The new experimental data were obtained by spatial spectroscopic technique using additional pulse helium puffing in hydrogen plasma. Simultaneously microscale plasma oscillations in the frequency band (0.01–2) MHz are observed with local enhanced microwave scattering diagnostics and by x-mode fluctuation reflectometry. Experiments demonstrate that the improved confinement is associated with the modification of microturbulence by the shear of theE×B poloidal velocity. This conclusion is also confirmed by the data obtained by Langmuir probes in the edge plasma.

Effect of the radial electric field on lower hybrid plasma heating in the FT-2 tokamak

Conditions for efficient ion heating in the interaction of lower hybrid waves with plasma are experimentally determined. Experiments show that efficient lower hybrid heating stimulates a transition to the improved confinement mode. The formation of internal and external transport barriers is associated with strong central ion heating, which results in a change of the radial electric field Er and an increase in the shear of the poloidal plasma velocity. The improved confinement mode in the central region of the discharge is attained under the combined action of lower hybrid heating and an additional rapid increase in the plasma current. A new mechanism for the generation of an additional field Er is proposed to explain the formation of a transport barrier.

Recent experiments with ECRH and ECCD in T-10

Experiments with ECRH performed during 1998-2000 are presented. An operating regime with an edgetransport barrier, interpreted as the H mode, has been studied with both on-axis and off-axis ECRH.Experimental data imply formation of an internal transport barrier simultaneously with the external transportbarrier in the case of off-axis ECRH. Strong evolution of plasma potential in the regions of the external andinternal transport barriers has been observed using heavy ion beam probe diagnostics. The mode of improvedconfinement with peaked density profiles and increased ion temperature has been observed after pellet injectioninto ECRH plasmas. Instabilities, identified as neoclassical tearing modes, have been found to limit β in the regimes with ahigh fraction of bootstrap current. The dependence of the critical β on the q(r) profile has been observed. Asystematic study of plasma turbulence has been started using correlation reflectometry diagnostics.

Investigation of the H-mode during ECRH in the T-10 tokamak

An improved confinement regime with an external transport barrier (H-mode) is obtained during electron-cyclotron resonance heating of a plasma in the T-10 tokamak. A characteristic feature of this regime is a spontaneous density growth accompanied by a drop in the intensity of Dα line and an increase in βp by a factor of ∼1.6. The threshold power for the L-H transition is close to that predicted by the ITER scaling. The best characteristics of the H-mode are achieved with decreasing qL to 2.2. It is shown that the external transport barrier arises for electrons, whereas the heat transport barrier insignificantly contributes to improved confinement.

Sensitivity of the plasma electric potential to the transport barriers formation

Heavy Ion Beam Probe diagnostics was used to study plasma potential and density in the regimes with improvement in confinement on the T-10 tokamak. Time-spatial potential distribution was analyzed during the formation of transport barriers. The time evolution of the potential profile shows simultaneous formation of the internal negative potential well and external layer with strong Er, during the simultaneous formation of the internal and external transport barriers. The time history of the plasma potential in the inner region (0.6<ρ<1) shows its clear correlation with theDα intensity.